The present invention relates generally to the synthesis of carbon nanotubes on substrates. More specifically, the invention relates to dense arrays of well-aligned carbon nanotubes filled with conductive filler synthesized on conductive substrates and a method for making these carbon nanotubes.
Carbon nanotubes constitute a new class of materials with a broad range of potential applications. Their unique properties make carbon nanotubes ideal candidates for novel application in areas such as vacuum microelectronics, flat panel displays, scanning probes and sensors, field emission devices and nanoelectronics.
A wide range of techniques has been used to prepare carbon nanotubes. For example, carbon nanotubes can now be produced in high yield and with reasonable quality as reported by C. Journet et al., Nature 388, 756 (1997), using arc discharge, by A. Thess et al., Science 273, 483 (1996), using laser ablation, and by R. T. Baker, Carbon 27, 315 (1989), using thermal decomposition of hydrocarbons.
Alignment of carbon nanotubes is particularly important for their use in applications such as flat panel displays. Ajayan et al., Science 265, 1212 (1994) report manufacturing a composite with carbon nanotubes randomly dispersed inside a polymer resin matrix and found that slicing the composite caused partial alignment of the nanotubes on the cut surface. De Heer et al., Science 268, 845 (1995) fabricated partially aligned nanotube films by drawing a nanotube suspension through a micropore filter.
More recently, well-aligned carbon nanotube arrays have been synthesized on solid substrates. W. Z. Li et al., Science 274, 1701 (1996), report well-aligned carbon nanotube arrays synthesized by thermal decomposition of acetylene gas in nitrogen on porous silica containing iron nanoparticles, and Z. F. Ren et al., Science 282, 1105 (1998), report well-aligned carbon nanotube arrays synthesized by hot-filament plasma-enhanced thermal decomposition of acetylene gas on nickel-coated glass. All of these preparations however, result in isolated carbon nanotubes on substrates where all the nanotubes are separated by distances on the order of 100 nanometers within the arrays. Disadvantages of these separations between the carbon nanotubes include decreased nanotube capacity on the substrate and a decreased ability to maintain alignment as the nanotubes grow longer.
Although hollow carbon nanotubes have substantial utility, it is recognized that filling the hollow core of carbon nanotubes with materials having useful physical, chemical, and electronic properties significantly broadens the range of potential applications for carbon nanotubes. Early attempts to fill carbon nanotubes were based on electric arc or laser ablation methods using metal/carbon composites as reported for example by Zhang et al., Science 281, 973 (1998), or on capillary-force infiltration of open-ended nanotubes as reported by Ugarte et al., Science 274, 1897 (1996). In addition, Dia et al., Nature 375, 769 (1995), reported an attempt to fill carbon nanotubes resulting in the reaction of the carbon nanotubes with titanium oxide (TiO) which converted all the nanotubes into titanium carbide (TiC) nanorods. In these and other prior experiments the carbon nanotubes were found to be only partially filled to a level of approximately 10%. The disadvantage of having carbon nanotubes that can only be partially filled is a decrease in the benefit sought to be gained through the useful properties of the materials filling the nanotube cores.
In view of the current and potential applications for carbon nanotubes, there remains a need in carbon nanotube technology for a method of synthesizing dense arrays of well-aligned carbon nanotubes on conductive substrates where the nanotubes are simultaneously and completely filled with conductive materials.
The present invention includes carbon nanotubes whose hollow cores are 100% filled with conductive filler. The carbon nanotubes are in uniform arrays on a prepared substrate and are well-aligned and can be densely packed. The uniformity of the carbon nanotube arrays is indicated by the uniform length and diameter of the carbon nanotubes, both which vary from nanotube to nanotube on a given array by no more than about 5%. The alignment of the carbon nanotubes is indicated by the perpendicular growth of the nanotubes from the prepared substrate which is achieved in part by the simultaneous growth of the conductive filler within the hollow core of the nanotube and the densely packed growth of the nanotubes. The present invention provides a densely packed carbon nanotube growth where each nanotube is in contact with at least one nearest-neighbor nanotube. The prepared substrate is a conductive substrate coated with a growth catalyst, and the conductive filler can be single crystals of carbide formed by a solid state reaction between the conductive substrate material and the growth catalyst.
The present invention further provides a method for making the filled carbon nanotubes on prepared substrates. The method includes the steps of depositing a growth catalyst onto a conductive substrate to form the prepared substrate, creating a vacuum within a vessel which contains the prepared substrate, flowing H2/inert (e.g. Ar) gas within the vessel to increase and maintain the pressure within the vessel, increasing the temperature of the prepared substrate, and changing the H2/Ar gas to ethylene gas such that the ethylene gas flows within the vessel. Additionally, varying the density and separation of the catalyst particles on the conductive substrate can be used to control the diameter of the nanotubes.
It is an object of the present invention to provide a method for the synthesis of dense arrays of well-aligned carbon nanotubes on prepared substrates where each carbon nanotube is simultaneously and completely filled with a conductive filler.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.